1. Field of the Invention
This invention relates to a filmless dental radiography system, and more particularly to a filmless dental radiography system that includes an intraoral radiation sensor that interfaces with the Universal Serial Bus (USB) port of a desktop, tower or portable (such as laptop or notebook) computer.
2. Description of the Related Art
Dentists and oral surgeons typically use x-radiation ("x-rays") to obtain images of their patients' teeth, mouths and gums to aid in diagnosis and treatment. In traditional oral and dental radiography, a cartridge containing a piece of photographic film is placed in the patient's mouth, for example behind a patient's tooth, and an x-ray beam is projected through the tooth and onto the film. The film, after being exposed in this manner, is developed in a dark room or a closed processor using special chemicals to obtain a photographic image of the tooth.
More recently, the field of filmless dental radiography has emerged. In filmless dental radiography, an x-ray beam is still projected through the patient's tooth, but no photographic film is used. Instead, an electronic sensor is placed in the patient's mouth behind the tooth to be examined. The electronic sensor may include a charge-coupled device (CCD), an active pixel sensor (APS) array or any other filmless radiation sensor. The x-rays pass through the tooth and impinge on the electronic sensor, which converts the x-rays into an electrical signal. The electrical signal is transmitted over a wire to a computer, either directly or though a module containing intermediate processing circuitry. The computer then processes the signal to produce an image on an associated output device, such as a monitor or a printer.
Filmless dental radiography offers several advantages over traditional film-based radiography. Most importantly, the electronic sensor is much more sensitive to x-rays than is film, allowing the dosage of x-rays to the patient to be lowered by as much as 90%. Also, the image of the tooth is generated by the computer almost instantaneously, thus eliminating the entire developing process, including the use of potentially harmful chemicals. In addition, because the images are generated electronically, they can be stored electronically in a computer database.
Examples of filmless dental radiography systems include those described in U.S. Pat. No. 4,160,997 to Robert Schwartz and U.S. Pat. No. 5,434,418 to David Schick. Filmless dental radiography systems typically utilize a standard desktop computer, such as an IBM or IBM compatible type personal computer. To provide a data path between the electronic sensor (or the intermediate module) and the computer's CPU, some conventional systems use the computer's Peripheral Component Interconnect (PCI) bus. The PCI bus, a internal 32-bit local bus that runs at 33 MHz and carries data at up to 133 megabytes per second (MBps). Other conventional filmless dental radiography systems use the computer's Industry Standard Architecture (ISA) bus, an 8- or 16-bit internal bus that carries data at up to 8.33 MBps. Each of these buses may act as a suitable interface between the sensor and computer.
While generally good for their intended applications, systems that use the computer's PCI or ISA bus have certain drawbacks. Most notably, the PCI and ISA buses are internal, and require that a specially designed circuit board be installed inside of the computer. The need for such a board increases the cost and reduces the reliability of the overall system.
Moreover, installing such a board is a time-consuming task that may only be performed by someone trained in the installation of computer peripherals. In particular, the installation requires the physical opening of the computer's housing, the clearing of any casing or wiring that may be in the way of the slot, the insertion of the card into the slot and the re-assembly of the housing once the insertion is complete. These are not tasks that are easily performed by the typical user of a filmless dental radiography system, such as a dentist, endodontist or oral surgeon.
In addition, many practitioners use a single sensor in conjunction with several computers, such as having a separate computer associated with each patient chair in the practitioners office. For such a scenario to be practical, a separate board must be installed into each of the computers, further increasing the cost of the overall system.
Moreover, the number of PCI and ISA slots available in a desktop or tower computer is limited. Installing a circuit board in a given slot to support a filmless dental radiography system precludes the use of that slot for some other type of peripheral device. Once all slots for a given bus are used, no more peripherals can be interfaced through that bus, unless one of the installed boards is removed and replaced with the board for the new peripheral. Such removal and replacement is not something that can be conveniently done on a regular basis.
Further still, portable computers, such as laptops and notebooks, generally are not provided with PCI or ISA slots. Accordingly, a conventional filmless dental radiography system cannot be used with such portable computers.
Very recently, desktop, tower and portable computers are being made available with a Universal Serial Bus (USB) port. The USB is a serial 12 megabit per second (Mbps) channel that can be used for peripherals. The USB is a token-based bus. In particular, the USB host controller broadcasts tokens on the bus and a device that detects a match on the address in the token responds by either accepting or sending data to the host. The host also manages USB bus power by supporting suspend/resume operations.
Unlike the PCI and ISA buses, the USB port does not require the use of a specially designed circuit board inside the computer. Accordingly, once the appropriate software has been installed, a peripheral simply need be plugged into the USB port to be ready for operation. In addition, one device can be unplugged and another plugged in without changing the hardware configuration of the computer.
Also, the USB port is "hot swappable," meaning that a first peripheral may be unplugged and a second peripheral plugged in without turning off and restarting the computer. In addition, the USB uses tiered star topology, allowing up to 127 different peripherals on the bus at a time. Further still, not only desktop and tower computers have USB ports; laptop and notebook computers are provided with USB ports as well.
While the USB port has received a great deal of attention from those designing computer peripherals as of late, no one has heretofore thought to use it as an interface for a filmless dental radiograph system. This is primarily because the USB is much slower than the PCI or ISA buses. More particularly, the theoretical maximum bandwidth of the USB is 12 Mbps (1.5 MBps), several times slower than the 8.33 MBps ISA bus and orders of magnitude slower than the 133 MBps PCI bus. And because many peripherals might be connected to the USB, no single peripheral can expect to realize the full range of the 1.5 MBps maximum theoretical bandwidth of the USB, making the practical bandwidth of the USB substantially less.
Accordingly, the USB is not believed to be fast enough to support the data flow requirements of a scientific sensor, such as a filmless dental radiography sensor. For example, in a conventional filmless dental radiography system analog data might be read-out of the sensor at a rate on the order of 4 million pixels per second (Mpps), converted on a real-time basis to digital data by an analog-to-digital converter (ADC) in an intermediate module and provided on a real-time basis to the computer's PCI or ISA bus. If a 16-bit (2 byte) ADC is used, an interface that can carry data at 8 MBps is required for such data transfer. This is several times greater than even the 1.5 MBps theoretical maximum bandwidth of the USB. Even a system which reads-out data at rate of 1 Mpps and uses a 12-bit (1.5 byte) ADC requires 1.5 MBps of bandwidth, the theoretical maximum bandwidth of the USB, and would strain or exceed the capabilities of the USB.
Some computer peripherals, such as digital cameras, have relatively low image quality requirements, and contend with the relatively smaller of the USB by simply reading-out data more slowly. This approach, however, is not suitable for a scientific sensor such a filmless dental radiography system, in which the quality of the image is paramount. In particular, a slower read-out rate results in a greater accumulation of dark signal (i.e. that part of the image data created by thermally generated electron-hole pairs) in the sensor, which results in turn in greater image degradation. Such results, while perhaps acceptable for a digital camera, are completely unacceptable for a scientific sensor such as a filmless dental radiography system, which must produce images of clarity sufficient to facilitate the diagnosis and treatment cavities, dental roots and the like.
There is a need, therefore, for a filmless dental radiography system that solves the inherent problems associated with the PCI and ISA buses by exploiting the advantages of using the USB port, while at the same time overcoming the obstacles that have heretofore prevented the USB port from being used for scientific sensors.